Many systems require a wiring and/or electrical contacts in order to supply electrical power to devices. Omitting these wires and contacts provides for an improved user experience. Traditionally, this has been achieved using batteries located in the devices but this approach has a number of disadvantages including extra weight, bulk and the need to frequently replace or recharge the batteries. Recently, the approach of using wireless inductive power transfer has received increasing interest.
Part of this increased interest is due to the number and variety of portable and mobile devices having exploded in the last decade. For example, the use of mobile phones, tablets, media players etc. has become ubiquitous. Such devices are generally powered by internal batteries and the typical use scenario often requires recharging of batteries or direct wired powering of the device from an external power supply.
As mentioned, most present day devices require a wiring and/or explicit electrical contacts to be powered from an external power supply. However, this tends to be impractical and requires the user to physically insert connectors or otherwise establish a physical electrical contact. It also tends to be inconvenient to the user by introducing lengths of wire. Typically, power requirements also differ significantly, and currently most devices are provided with their own dedicated power supply resulting in a typical user having a large number of different power supplies with each power supply being dedicated to a specific device. Although, internal batteries may prevent the need for a wired connection to an external power supply, this approach only provides a partial solution as the batteries will need recharging (or replacing which is expensive). The use of batteries may also add substantially to the weight and potentially cost and size of the devices.
In order to provide a significantly improved user experience, it has been proposed to use a wireless power supply wherein power is inductively transferred from a transmitter coil in a power transmitter device to a receiver coil in the individual devices.
Power transmission via magnetic induction is a well-known concept, mostly applied in transformers which have a tight coupling between the primary transmitter coil and the secondary receiver coil. By separating the primary transmitter coil and the secondary receiver coil between two devices, wireless power transfer between the devices becomes possible based on the principle of a loosely coupled transformer.
Such an arrangement allows a wireless power transfer to the device without requiring any wires or physical electrical connections. Indeed, it may simply allow a device to be placed adjacent to, or on top of, the transmitter coil in order to be recharged or powered externally. For example, power transmitter devices may be arranged with a horizontal surface on which a device can simply be placed in order to be powered.
Furthermore, such wireless power transfer arrangements may advantageously be designed such that the power transmitter device can be used with a range of power receiver devices. In particular, a wireless power transfer standard known as the Qi standard has been defined and is currently being developed further. This standard allows power transmitter devices that meet the Qi standard to be used with power receiver devices that also meet the Qi standard without these having to be from the same manufacturer or having to be dedicated to each other. The Qi standard further includes some functionality for allowing the operation to be adapted to the specific power receiver device (e.g. dependent on the specific power drain).
The Qi standard is developed by the Wireless Power Consortium and more information can e.g. be found on their website: http://www.wirelesspowerconsortium.com/index.html, where in particular the defined Standards documents can be found.
In order to support the interworking and interoperability of power transmitters and power receivers, it is preferable that these devices can communicate with each other, i.e. it is desirable if communication between the power transmitter and power receiver is supported, and preferably if communication is supported in both directions.
The Qi standard supports communication from the power receiver to the power transmitter thereby enabling the power receiver to provide information that may allow the power transmitter to adapt to the specific power receiver. In the current standard, a unidirectional communication link from the power receiver to the power transmitter has been defined and the approach is based on a philosophy of the power receiver being the controlling element. To prepare and control the power transfer between the power transmitter and the power receiver, the power receiver specifically communicates information to the power transmitter.
The unidirectional communication is achieved by the power receiver performing load modulation wherein a loading applied to the secondary receiver coil by the power receiver is varied to provide a modulation of the power signal. The resulting changes in the electrical characteristics (e.g. variations in the current draw) can be detected and decoded (demodulated) by the power transmitter. In this approach, the power transfer signal is essentially used as a carrier which is modulated by the power receiver, i.e. by modulating a load on the power receiver coil by e.g. switching on and off an impedance that is connected to the power receiver coil.
However, a limitation of the Qi system is that it does not support communication from the power transmitter to the power receiver. In order to address this, various communication approaches have been proposed. For example, it has been proposed to communicate data from the power transmitter to the power receiver by modulating the power transfer signal with a suitable signal representing the data to be transmitted. E.g. small frequency variations representing the data may be superposed on the power transfer signal.
In general, communication between power receiver and power transmitter is faced with multiple challenges and difficulties. In particular, there is typically a conflict between the requirements and characteristics for the power signal in transferring power and the requirements and preferences for the communication. Typically, the system requires close interaction between the power transfer and communication functions. For example, the system is designed based on the concept of only one signal being inductively coupled between the transmitter and the power receiver, namely the power signal itself. However, using the power signal itself for not only performing a power transfer but also for carrying information results in difficulties due to the varying operating characteristics.
As a specific example, using a load modulation approach wherein the power receiver communicates data by modulating the load of the power signal (such as in the Qi system) requires that the normal load is relatively constant. However, this cannot be guaranteed in many applications.
E.g., if wireless power transfer is to be used to power a motor driven appliance (such as e.g. a blender), the motor current tends to be quite erratic and discontinuous. Indeed, when a motor driven appliance draws current, the amplitude of the current is strongly related to the load of the motor. If the motor load is changing, the motor current is changing as well. This results in the amplitude of the current in the transmitter also changing with the load. This load variation will interfere with the load modulation, resulting in degraded communication. Indeed, in practice it is typically very difficult to detect load modulation for loads that include a motor as part of the load. Therefore, in such scenarios, the number of communication errors is relatively high or the communication may utilize a very high data symbol energy, thereby reducing the possible data rate very substantially.
In order to address the problems with load modulation, it has been proposed to use a separate and independent communication link from the power receiver to the power transmitter. Such an independent communication link may provide a data path from the power receiver to the power transmitter which is substantially independent of the power transfer operation and dynamic variations. It may also provide a higher bandwidth and often a more robust communication.
However, there are also disadvantages associated with using an independent communication link. For example, the use of separate communication channels could result in interference between the operations of different power transfers which could result in a potentially dangerous situation with high power levels. For example, the control operations may interfere with each other, e.g. by the control data from the power receiver of one power transfer operation being used to control the power transfer to another nearby power receiver The separation between communication and power transfer signals may result in less robust and less fail safe operation.
Another potential problem with wireless power transfer is that power may unintentionally be transferred to unintended e.g. metallic objects. For example, if a foreign object, such as e.g. a coin, key, ring etc., is placed upon the power transmitter platform arranged to receive a power receiver, the magnetic flux generated by the transmitter coil will introduce eddy currents in the metal objects which will cause the objects to heat up. The heat increase may be very significant and may indeed result in a risk of pain and damage to humans subsequently picking up the objects.
Experiments have shown that metal objects positioned at the surface of a power transmitter can reach an undesired high temperature (higher than 60° C.) at normal environment temperatures (20° C.) even for power dissipation in the object being as low as 500 mW. For comparison, skin burning caused by contact with hot objects starts at temperatures of around 65° C. The experiments have indicated that a power absorption of 500 mW or more in a typical foreign object rises its temperature to an unacceptable level.
In order to prevent such scenarios, it has been proposed to introduce foreign object detection where the power transmitter can detect the presence of a foreign object and reduce the transmit power. For example, the Qi system includes functionality for detecting a foreign object, and for reducing power if a foreign object is detected.
The power dissipation in a foreign object can be estimated from the difference between transmitted and received power. In order to prevent that too much power is dissipated in a foreign object, the transmitter can terminate the power transfer if the power loss exceeds a threshold.
In the current Qi Standard the preferred approach is to determine the power loss across the interface between the power transmitter and the power receiver in order to determine any loss in foreign objects. For this purpose, the power receiver estimates the amount of power that enters its interface surface—i.e. the received power. In order to generate the estimate, the power receiver measures the amount of power provided to the load, and adds an estimate of the losses in components—coil, resonant capacitor, rectifier, etc., as well as losses in conductive elements of the device, such as in metal parts that are not exposed to the user. The power receiver communicates the determined received power estimate to the power transmitter at regular intervals.
The power transmitter estimates the amount of power extracted from the power signal—i.e. the transmitted power. The power transmitter can then calculate the difference between the transmitted power and the received power, and if the difference exceeds a given level, the power transmitter may determine that a situation has occurred where an unacceptable power may be dissipated in a foreign object. For example, a foreign object may be positioned on or near the power transmitter resulting in this being heated due to the power signal. If the power loss exceeds a give threshold, the power transmitter terminates the power transfer in order to prevent the object from getting too hot. More details can be found in the Qi Standard, System Description Wireless power.
When performing this power loss detection, it is important that the power loss is determined with sufficient accuracy to ensure that the presence of a foreign object is detected. Firstly, it must be ensured that a foreign object which absorbs significant power from the magnetic field is detected. In order to ensure this, any error in estimating the power loss calculated from the transmitted and received power must be less than the acceptable level for power absorption in a foreign object. Similarly, in order to avoid false detections, the accuracy of the power loss calculation must be sufficiently accurate to not result in estimated power loss values that are too high when no foreign object is present.
It is substantially more difficult to determine the transmitted and received power estimates sufficiently accurately at higher power levels than for lower power levels. For example, assuming that an uncertainty of the estimates of the transmitted and received power is ±3%, this can lead to an error of                ±150 mW at 5 W transmitted and received power, and        ±1.5 W at 50 W transmitted and received power.        
Thus, whereas such accuracy may be acceptable for a low power transfer operation it is not acceptable for a high power transfer operation.
Typically, it is required that the power transmitter must be able to detect power consumption of foreign objects of only 350 mW or even lower. This requires very accurate estimation of the received power and the transmitted power. This is particularly difficult at high power levels, and frequently it is difficult for power receivers to generate estimates that are sufficiently accurate. However, if the power receiver overestimates the received power, this can result in power consumption by foreign objects not being detected. Conversely, if the power receiver underestimates the received power, this may lead to false detections where the power transmitter terminates the power transfer despite no foreign objects being present.
Thus, the current approaches for foreign object detection and communication may be suboptimal and have some associated disadvantages.
Accordingly, an improved power transfer system would be advantageous and in particular a system allowing improved communication support, increased reliability, increased flexibility, facilitated implementation, reduced sensitivity to load variations, improved safety, improved foreign object detection, and/or improved performance would be advantageous.